x86/iommu: check for CMPXCHG16B when enabling IOMMU

All hardware with VT-d/AMD-Vi has CMPXCHG16B support. Check this at
initialisation time, and otherwise refuse to use the IOMMU.

If the local APICs support x2APIC mode the IOMMU support for interrupt
remapping will be checked earlier using a specific helper.  If no support
for CX16 is detected by that earlier hook disable the IOMMU at that point
and prevent further poking for CX16 later in the boot process, which would
also fail.

There's a possible corner case when running virtualized, and the underlying
hypervisor exposing an IOMMU but no CMPXCHG16B support.  In which case
ignoring the IOMMU is fine, albeit the most natural would be for the
underlying hypervisor to also expose CMPXCHG16B support if an IOMMU is
available to the VM.

Note this change only introduces the checks, but doesn't remove the now
stale checks for CX16 support sprinkled in the IOMMU code.  Further changes
will take care of that.

Suggested-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Teddy Astie <teddy.astie@vates.tech>
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
master commit: 2636fcdc15c707d5e097770133f0afb69e8d70c9
master date: 2025-01-27 13:05:11 +0100

diff --git a/xen/drivers/passthrough/amd/iommu_intr.c b/xen/drivers/passthrough/amd/iommu_intr.c
index 7fc796dec25bf9719940f4b143d5008aad043b7e..f07fd9e3d97043898eb4e0b89b9054edc05eefb6 100644 (file)
--- a/xen/drivers/passthrough/amd/iommu_intr.c
+++ b/xen/drivers/passthrough/amd/iommu_intr.c
@@ -649,6 +649,19 @@ bool __init cf_check iov_supports_xt(void)
     if ( !iommu_enable || !iommu_intremap )
         return false;
 
+    if ( unlikely(!cpu_has_cx16) )
+    {
+        AMD_IOMMU_ERROR("no CMPXCHG16B support, disabling IOMMU\n");
+        /*
+         * Disable IOMMU support at once: there's no reason to check for CX16
+         * yet again when attempting to initialize IOMMU DMA remapping
+         * functionality or interrupt remapping without x2APIC support.
+         */
+        iommu_enable = false;
+        iommu_intremap = iommu_intremap_off;
+        return false;
+    }
+
     if ( amd_iommu_prepare(true) )
         return false;
 

diff --git a/xen/drivers/passthrough/amd/pci_amd_iommu.c b/xen/drivers/passthrough/amd/pci_amd_iommu.c
index 73dcc4a2dd0ceed215e1b09258d0e4363bc2dbd0..f96f59440bcc8faecd46f8267b5d25bff99f6ce5 100644 (file)
--- a/xen/drivers/passthrough/amd/pci_amd_iommu.c
+++ b/xen/drivers/passthrough/amd/pci_amd_iommu.c
@@ -309,6 +309,12 @@ static int __init cf_check iov_detect(void)
     if ( !iommu_enable && !iommu_intremap )
         return 0;
 
+    if ( unlikely(!cpu_has_cx16) )
+    {
+        AMD_IOMMU_ERROR("no CMPXCHG16B support, disabling IOMMU\n");
+        return -ENODEV;
+    }
+
     if ( (init_done ? amd_iommu_init_late()
                     : amd_iommu_init(false)) != 0 )
     {

diff --git a/xen/drivers/passthrough/vtd/intremap.c b/xen/drivers/passthrough/vtd/intremap.c
index c504852eb818b1e0fda6bc83bfa198dd1f899ae8..233db5cb64de5ce2635ba713fd5f327d468ecc20 100644 (file)
--- a/xen/drivers/passthrough/vtd/intremap.c
+++ b/xen/drivers/passthrough/vtd/intremap.c
@@ -150,6 +150,19 @@ bool __init cf_check intel_iommu_supports_eim(void)
     if ( !iommu_qinval || !iommu_intremap || list_empty(&acpi_drhd_units) )
         return false;
 
+    if ( unlikely(!cpu_has_cx16) )
+    {
+        printk(XENLOG_ERR VTDPREFIX "no CMPXCHG16B support, disabling IOMMU\n");
+        /*
+         * Disable IOMMU support at once: there's no reason to check for CX16
+         * yet again when attempting to initialize IOMMU DMA remapping
+         * functionality or interrupt remapping without x2APIC support.
+         */
+        iommu_enable = false;
+        iommu_intremap = iommu_intremap_off;
+        return false;
+    }
+
     /* We MUST have a DRHD unit for each IOAPIC. */
     for ( apic = 0; apic < nr_ioapics; apic++ )
         if ( !ioapic_to_drhd(IO_APIC_ID(apic)) )

diff --git a/xen/drivers/passthrough/vtd/iommu.c b/xen/drivers/passthrough/vtd/iommu.c
index e13be244c1305735fa5b0b27df2d256896e9f2aa..ab38bbc8a5684c70b4801a3d3c7e15c7258da2bd 100644 (file)
--- a/xen/drivers/passthrough/vtd/iommu.c
+++ b/xen/drivers/passthrough/vtd/iommu.c
@@ -2630,6 +2630,13 @@ static int __init cf_check vtd_setup(void)
     int ret;
     bool reg_inval_supported = true;
 
+    if ( unlikely(!cpu_has_cx16) )
+    {
+        printk(XENLOG_ERR VTDPREFIX "no CMPXCHG16B support, disabling IOMMU\n");
+        ret = -ENODEV;
+        goto error;
+    }
+
     if ( list_empty(&acpi_drhd_units) )
     {
         ret = -ENODEV;